This invention relates to detector assemblies for in-core nuclear reactor monitoring.
There are now a significant number of nuclear power plants operating and under construction in this country and about the world as designed by various manufacturers and others. With the current climate regarding nuclear power plants and the apprehension thereof (unfounded or not), the rate at which any additional plants will be planned, in this country at least, is expected to be minimal for at least the near future.
The operating plants and those fairly well along are constantly subject to being improved or upgraded in different ways from time to time. The desire to improve such plants is sometimes apparently frustrated by established design parameters based upon what was earlier known and thought desirable at the time. This can happen by way of established dimensions, for example, which would seem to preclude doing certain things because of dimensional constraints.
The assignee of this patent application is, and has been a leader in the number of nuclear plants it has designed or which have devolved from its design. By far the majority of the plants of assignee have a design in which thimble guide tubes, through which in-core thimbles are inserted and retracted, have an inside diameter of about 0.0102 m, although a relatively few plants have thimble guide tubes of about 0.0112 m internal diameter. The in-core thimbles, which are frequently called retractable thimbles, have their diametric dimension established from the dimensions of the fuel assembly in which the thimble guide tube is inserted (and from which it is retracted as during refueling). The transverse cross-sectional dimensions of the fuel assembly are basically standardized because of the dimensional standardization of the components which make up the total cross-section of the assembly.
Typically, the in-core thimbles in the plants of assignee have been used to permit the insertion of movable miniature detectors which are driven through this thimble to scan the active lengths of the particular fuel assembly in which the thimble is situated. The in-core flux thimble has a nominal outside diameter (OD) of 0.0076 m and a minimum inside diameter (ID) of 0.0050 m, and serves as the high pressure barrier to reactor coolant, meeting the ASME code requirements for normal pressurized water reactor in-core environmental condition of 2500 psia (17.23 E+6 Pa) at 650.degree. F. (343.degree. C.). A standard movable miniature detector for such a thimble has an OD of 0.0048 m and is welded on the end of a hollow, helically wrapped drive cable having the same nominal OD as the detector.
The cable drive systems for these installations are fairly complicated in the sense of their abilities through various transfer and switching devices to change routings of the detectors through, say, about 60 different routes, and to accomplish other desired results. Accordingly, these drive systems are relatively expensive.
Nuclear plants designed by other than the assignee of this invention may have guide tubes and thimbles dimensionally larger than the great majority of the assignee designed plants so that dimensional problems are largely avoided.
The existing arrangement of assignee designed plants, in which a thimble is simply used to provide a passage for a movable detector, does not impose any dimensional constraints. Nevertheless, there is now a move toward providing more instrumentation in the instrumentation locations available, that is, at the locations where thimble tubes are inserted. Such additional instrumentation typically comprises stationary, self-powered neutron detectors and thermocouples. The additional instrumentation does not avoid the requirement that movable detectors for scanning the height of the core also be available.
The additional instrumentation plus the movable detector is provided, when dimensionally permitted for a particular plant, in an arrangement of coaxial inner and outer tubes. The inner tube is called a calibration tube because a movable detector is adapted to be moved there along to obtain readings at different locations. The ID of the outer tube (which is sometimes called a housing tube), is sufficiently larger than the OD of the calibration tube that the annular space therebetween is of sufficient transverse dimension to accommodate stationary thermocouples and self-powered detectors therein.
With the single tube or "bare" thimble, which provides only for passage of a movable detector, the thimble tube provides the required pressure barrier. With the multi-instrumentation concentric tube arrangement, the inner calibration tube has been selected as the sole element to meet the pressure--temperature requirements so far as we know and, accordingly, its wall thickness becomes one of the significant factors in the effort to package the desired stationary instrumentation, along with having an adequate ID of the calibration tube to permit passage of a movable detector.
For those relatively few plants designed by assignee which have the 0.0112 m ID guide tubes, or other plants having equal or larger guide tubes, no particular problem is experienced since the OD of the outer housing tube is less than the 0.0112 m ID of the guide tube, the OD of the housing tube being, say, 0.0107 m.
For the great majority of plants designed by assignee in which the guide tubes are 0.0102 m ID, and which have the basically standard drive systems with movable detectors and cables of 0.0048 m OD, a serious problem exists in that the ID of the calibration tube (about 0.0035 m) is less than the OD of the standard movable detector and the cable of the drive system. One way to solve this problem is to replace larger (0.0048 m) movable detectors with smaller diameter detectors of about 0.0030 m OD and to replace the standard size cable drive system with a new drive system having a cable with a 0.0030 m OD and associated components sized for the smaller cable. While this solution to the problem is conceptually simple, and has in fact been used, the added expense per plant is about one quarter million dollars. Further, the smaller diameter detector and drive system therefor is considered to be somewhat less reliable than the larger size original.
The aim of the invention is to provide a concentric thimble arrangement, especially for assignee designed plants having the smaller diameter guide tubes, in which the additional instrumentation desired can be accommodated, the existing larger diameter standard size movable detectors and cable systems can be used, and the concentric tube arrangement will meet pressure barrier code requirements for reactor environmental conditions of 17.23 E+6 Pa at 343.degree. C.